1. Field of the Invention
This invention relates to aircraft infrared background matching in which the upper surface of an aircraft skin is electrically heated to provide an infrared contrast signature matching the earth background.
2. Background of the Invention
An aircraft in flight is generally susceptible to detection from above by an infrared (IR) sensor that senses the contrast in temperature between the earth background and the aircraft skin. Aircraft operate at different power settings, such as, cruise, military and afterburner while on a mission and each setting dictates different power flight conditions. The power conditions include velocity, altitude and power, which affect the temperature of the aircraft skin resulting in an aircraft IR signature.
The power flight conditions for an aircraft flying at cruise include high altitude and low Mach number. Mach number is the ratio of velocity to the speed of sound. During this setting, the aircraft skin approaches the relatively low temperature of the surrounding atmosphere. Consequently, when viewed from an above platform, such as missles, aircraft and spacecraft, the subject aircraft will be flying over the relatively warmer earth background resulting in a negative IR contrast signature. The aircraft will them become susceptible to detection by an IR detection system employed on the above platform. The negative IR contrast signature arises from the radiance emitted by the aircraft being less than the radiance of the background.
Since aircraft operate the majority of the time in cruise, it is desirable to provide a means for matching the IR signature of the aircraft upper skin with the associated terrain and sky background to minimize the contrast signature and thereby reduce the possibility of detection by an IR sensor.
One prior art method of background matching is disclosed in German Patent No. 2848072 wherein a radiometer is used to measure parameters such as temperature, visibility and cloud cover above an object in order to determine the background radiance. The background radiance is then used to control a microwave source incorporated in the object to provide an output that, coupled with the microwave radiation of the object, matches that reflected by the sky within the visibility range. The matching of the microwave background radiation system would not be suitable for contrast matching of an aircraft in flight where the aircraft and background radiance are continually changing.
U.S. Pat. No. 4,413,668 discloses use of forced air passing over the outer surface of an object to reduce the temperature and thus the thermal signature of the object. Air entrainment channels are attached to the surface of the object and together with multi-layered suppressors collect and expel the heat emanating from the object. The system would probably be too bulky and costly to be suitable for use on an aircraft.